So what happened? Most viewers blame the fact that Clash of the Titans was never intended to be a three-dimensional movie and that it was converted from a flat-image format at the last second. It's clear that Warner Bros. invested millions in the upgrade to capture the stupendous success of Avatar and Alice in Wonderland—giga-blockbusters that rode high on premium ticket prices and the promise of awesome spectacle. But the new film has critics grumbling over studio greed: It's been "3-D-ized on the cheap," they say, with a " 'quickie' conversion" that "few moviegoers will think [is] worth the extra bucks."

The studios are desperate to avoid this scenario. If the price of 3-D goes up while the quality goes down, consumers will abandon the format altogether—as they did in the 1950s and the 1980s. An abomination like Clash of the Titans—with ugly, pointless stereo effects—won't destroy the medium on its own, but a slew of movies like it all in a row could be a disaster. That's why executives are so keen on maintaining the "Hollywood-quality experience" of 3-D, says Phil Lelyveld, director of an industry-funded laboratory devoted to that very mission. "It's a new art form. We don't want to kill it before we figure out how to use it."

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So here's the urgent challenge for Lelyveld and the studio execs: How do you know what works in 3-D and what doesn't? Right now there are no rules for how to shoot 3-D movies, and no industry standards on how to project them. Directors without any experience in stereography are being asked to learn new technologies on the fly. And we still have only the barest understanding of why some depth effects hurt our eyes and others don't. Amid this chaos, a movie like Clash of the Titans might offer some precious guidance: What, exactly, went so wrong?

The first, and most obvious explanation is the one we've heard already—the cheapo postproduction upgrade. Hollywood directors are already moaning over such conversions. James Cameron, who spent four years in production on Avatar, has warned that half-assed 3-D could set back the whole medium. In an interview with Deadline.com, Michael Bay said he hates the look of upgraded footage and declared that "you can't just shit out a 3-D movie."

For all that, you didn't hear much complaining about Alice in Wonderland, which was also pushed to 3-D in post. What's the difference between Alice and Clash? It could well be the quality of the conversion. To add depth to a flat image, you need more than just a sophisticated computer program. Skilled 3-D artisans cost money, which is why these upgrades cost a reported $40,000 to $150,000 per minute to complete. If the conversion on Clash was done quickly, and for relatively little money, that might explain some of the "cardboard cutout effect" in the final product.

There's at least one more major difference between the two films. Clash of the Titans has plenty of CGI special effects, but most of the film is live-action—which poses special problems for stereo cinema. Alice in Wonderland, by contrast, is mostly animated, as are all the other recent 3-D blockbusters, like Up, Monsters vs. Aliens, Ice Age 3, A Christmas Carol, and Cloudy With a Chance of Meatballs. Even big-daddy Avatar, which some took as a test-case for live-action 3-D, was more animated than not. (In February, I argued that it should have been nominated for best animated feature.)

There are plenty of reasons why animated films might have a leg up when it comes to 3-D. For one, the movie-makers have full control over all the pixels on the screen, which allows for precise correction of every optical artifact. They also have access to a set of 3-D tricks that haven't yet migrated into live cinema. One nifty innovation, called the "dynamic floating window," tilts the edges of the frame forward and backward in space as the movie goes along. Those adjustments—invisible to the audience—let the director add depth to a scene without resorting to awkward, pop-out effects. According to Lelyveld, the animators are just "further along the learning curve" than conventional cinematographers.

Other flaws with live-action 3-D may be harder to fix. Among the worst is the "puppet theater effect," wherein a scene starts to look like an architect's model, with miniature bodies and bonsai trees posed in an artificial landscape. When ESPN started shooting football games in stereo, the puppet effect turned up right away: Slate's Justin Peters described the sense of "watching a living diorama—a bunch of humanoid action figures running around and tackling each other." Same goes for the medium and long shots in Clash of the Titans, where Perseus and company seemed shrunken down, at times, by some Olympian spell.

The puppet effect isn't much of a problem for animated movies, since everything is so stylized that distortions of scale hardly matter. In Coraline, a stop-motion film that was shot with live cameras, the puppet-theater effect was perfectly appropriate, since the characters were, in fact, puppets. There were some puppet-y moments in Avatar, too—think of the scene where Colonel Quaritch briefs the new recruits ("You're not in Kansas anymore"). But in a movie where most of the characters were supposed to be 10 feet tall, a bit of funny scaling hardly mattered.

Even if you forgive the puppet effect and a lousy conversion from 2-D to 3-D, Clash of the Titans appears to suffer for being shot with a flat image in mind. Stereo cinema has its own rules for visual storytelling, and some tried-and-true flat-film techniques are a liability in three dimensions. Quick cuts and fast-paced action scenes, for example, can be hard to follow in a 3-D movie. Some stereographers also try to avoid a shallow depth of field or focus pulls on the theory that people looking at 3-D like to scan around the image and become frustrated when things aren't in focus. (Here's another advantage for animation: Everything can be in focus at all times.) For this viewer, at least, the shallow focus in Clash was a painful distraction.

These problems aren't specific to last-second upgrades. Consider that almost every 3-D movie that's released is shown in both 2-D and 3-D theaters. Whether the movies are shot for stereo or flat, some significant proportion of the audience will be seeing it in the wrong format. From the perspective of maintaining "Hollywood-quality experience," these conflicts could be a serious setback. Lelyveld suggests that movies shot in 3-D might need to be recut for 2-D showings and vice-versa.

Then there's a whole raft of other variables that might ruin a 3-D movie for individual viewers. I've already gone over the potential for eyestrain: Stereo cinema is harder to watch than flat cinema, no matter how well it's made. Something like 1 in 25 viewers are stereo-blind, meaning they can't discern the depth effect at all, and another 5 to 10 percent of the population may be especially prone to headaches and nausea. Age matters, too—older viewers may have more trouble adjusting to the stereo image. Even details as seemingly insignificant as where you happen to be sitting in the movie theater can alter the apparent quality of a 3-D film.

Any of these factors could affect whether you enjoy Clash of the Titans and the way you respond to its 3-D effects. The trick, says Lelyveld, is for the studios to figure out how the experience can be optimized for as many people as possible. That's the problem Hollywood faced in the 1950s and the 1980s, too. Then, as now, there were calls for industry standards (PDF) for 3-D cinema. No one wanted a bunch of poorly-made B-movies to define the medium. But that's exactly what happened.